Waku v2 RLN Relay

The current specification embodies the details of the spam-protected version of relay protocol empowered by Rate Limiting Nullifiers (RLN). More details on RLN can be found in this spec (TODO: to link the spec).

The security objective is to control the number of PubSub messages that each peer can publish per epoch (where epoch is a system design parameter), regardless of the published topic.

Protocol identifier*: /vac/waku/waku-rln-relay/2.0.0-alpha1

Motivation #

In open p2p messaging networks, one big problem is spam-resistance. Existing solutions, such as Whisper’s proof of work, are insufficient, especially for heterogeneous nodes. Other reputation-based approaches might not be desirable, due to issues around arbitrary exclusion and privacy.

We augment the relay protocol with a novel, light, and effective spam prevention mechanism which also suits the resource-constrained nodes.

TODO: Fill in more

Flow #

SetUp and Registration #

A peer willing to publish a message is required to register. Registration is moderated through a smart contract deployed on the Ethereum blockchain. The state of the contract contains the list of registered members (realized by a Merkle Tree). An overview of registration is illustrated in Figure 1.

For the registration, a peer creates a transaction that sends x (TODO to be specified) ETH to the contract. The peer who has the “private key” sk associated with that deposit would be able to withdraw x ETH by providing valid proof. Note that sk is initially only known by the owning peer however it may get exposed to other peers in case the owner attempts spamming the system i.e., sending more than one message per epoch.

TODO: the interaction with the contract is subject to change depending on the final implementation

Once registered, the peer obtains the root of the tree (after the registration of the current peer) i.e., root as well as the authenticity path authPath. A peer can prove her membership using the authPath.

sk and authPath are secret data and MUST be permanently and locally stored by the peer.

TODO: To specify the details of protobuf messages for the interaction with the contract

TODO: the function calls in this figure as well as messages are subject to change

Publishing #

In order to publish at a given epoch, the publishing peer proceeds based on the regular relay protocol. However, in order to protect against spamming, each PubSub message must carry a proofBundle. At a high level, the proofBundle is a zero-knowledge proof (TODO: to clarify what a zero-knowledge proof means) signifying that the publishing peer is a registered member, and she has not exceeded the messaging rate at the given epoch.

The proofBundle is embedded inside the data field of the PubSub message, which, in the relay protocol, corresponds to the WakuMessage. More details on the proofBundle’s message fields are provided under the Protobuf section.

The proof generation relies on the knowledge of sk and authPath (that is why they should be permanently and privately stored by the owning peer). Further inputs to the proof generation are root, epoch and payload||contentTopic where payload and contentTopic come from the WakuMessage (TODO: the inputs of the proof generation may change). The proof generation results in the following data items which are included as part of the ProofBundle:

  1. shareX
  2. shareY
  3. nullifier
  4. zkProof

The tuple of (nullifier, shareX, ShareY) can be seen as partial disclosure of peer’s sk for the intended epoch. Given two such tuples with identical nullifier but distinct shareX, ShareY results in full disclosure of peer’s sk and hence burning the associated deposit. Note that the nullifier is a deterministic value derived from sk and epoch therefore any two messages issued by the same peer (i.e., sing the same sk) for the same epoch are guaranteed to have identical nullifiers.

Note that the authPath of each peer depends on the current status of the registration tree (hence changes when new peers register). As such, it is recommended (and necessary for anonymity) that the publisher updates her authPath based on the latest status of the tree and attempts the proof using her updated authPath.

Routing #

Upon the receipt of a PubSub message, the routing peer needs to extract and parse the proofBundle from the data field. If the epoch attached to the message has a non-reasonable gap (TODO: the gap should be defined) with the routing peer’s current epoch then the message must be dropped (this is to prevent a newly registered peer spamming the system by messaging for all the past epochs). Furthermore, the routing peers MUST check whether the proofBundle is valid and the message is not spam. If both checks are passed successfully, then the message is relayed. If proofBundle is invalid then the message is dropped. If spamming is detected, the publishing peer gets slashed. An overview of routing procedure is depicted in Figure 2.

Spam Detection and Slashing #

In order to enable local spam detection and slashing, routing peers MUST record the nullifier, shareX, and shareY of any incoming message conditioned that it is not spam and has valid proof. To do so, the peer should follow the following steps.

  1. The routing peer first verifies the zkProof and drops the message if not verified.
  2. Otherwise, it checks whether a message with an identical nullifier has already been relayed.
    1. If such message exists and its shareX and shareY components are different from the incoming message, then slashing takes place (if the shareX and shareY fields of the previously relayed message is identical to the incoming message, then the message is a duplicate and shall be dropped).
    2. If none found, then the message gets relayed.

An overview of slashing procedure is provided in Figure 2.

TODO: may shorten or delete the Spam detection and slashing process

TODO: may consider validator functions or extended validators for the spam detection

TODO: the function calls in this figure as well as messages are subject to change

Security Considerations #

TODO: add discussion about the anonymity (e.g., the StrictNoSign policy)

TODO: discuss about the economic spam guarantees

Protobuf #

//TODO may include the pubsub message 
// TODO to reflect this change on WakuMessage spec once the PR gets mature
message WakuMessage {
  optional bytes payload = 1;
  optional string contentTopic = 2;
  optional uint32 version = 3;
  optional ProofBundle proofBundle = 4;

message ProofBundle {
   int64 epoch = 1; //  indicating the intended epoch of the message
   // TODO shareX and shareY
   bytes nullifier = 2;
   bytes root = 3; // TODO may be removed and added as part of zkProof
   // TODO zkProof
// TODO ZKProof may be a separate message type
// TODO the protobuf messages for communicating with the contract

TODO: to describe ProofBundle message fields

Copyright #

Copyright and related rights waived via CC0.